1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly, it relates to an erbium doped fiber amplifier (EDFA) and its method of operation. The EDFA provided by this invention automatically traces transmitted light wavelengths and adjustably filters the wavelengths of light signals to be transmitted by adjusting the central wavelength of an optical filter installed at the output terminal of the amplifier, thereby preventing propagation of noise caused by the properties of the optical amplifier.
2. Description of the Related Art
When a transmission terminal in an optical communication network converts an electric signal into a light signal, and transmits it to a desired destination using optical fiber, EDFA are usually used to amplify the weakened light signals at predetermined distances along the transit route. This practice of periodic re-amplification ensures the transmission of stable signals. Such amplifiers are also typically installed in reception and transmission terminals to amplify electric power and perform pre-amplification.
An EDFA commonly includes a tunable filter that removes noise introduced into the amplified light signal during the amplification process. Such a filter has a central wavelength at which a light signal passed through the filter receives the least attenuation. Signal components at different wavelengths receive greater attenuation the farther those wavelengths are from the filter's central wavelength. When the filter is tuned for a central wavelength equal to the nominal wavelength of an incoming light signal, noise introduced by the amplifier can be efficiently eliminated from the amplified signal.
However, light signals received over optical communication networks do not always meet nominal parameters. In practice, such a signal will vary according to characteristics of the various components of the optical amplification device that generated or boosted it. Thus, transmitted light signal wavelengths may change as the optical amplification device operates over an extended period of time. They can also be influenced by ambient temperatures. To compensate for these effects, a wave-length-fixing type filter or a convertible-to-manual filter may be utilized. However, these approaches generally result in a loss of signal strength when instantaneous changes occur in the wavelength of the received signal. Also, they potentially create problems by decreasing the intensity of the light signals generated by the amplification device.
The use of tunable filters to remove noise introduced into optical communications signals by optical amplifiers is well known in the literature. For example, U.S. Pat. No. 4,945,531 provides a system with several tunable filters interposed between an optical wavelength demultiplexer and a multiplexer to filter spontaneous emission noise from around several channels in a WDM communications signal. U.S. Pat. No. 5,644,423 also discloses an amplifier for WDM signals that automatically adjusts a tunable filter central wavelength to trace the wavelength of a selected channel for gain control. This latter system constitutes a significant advance in EDFA filtering technology, but it requires a complicated wavelength feedback system that includes a reference oscillator and a synchronous detector to generate a wavelength error signal.
U.S. Pat. No. 5,570,221, entitled "Light Amplification Device" and issued Oct. 29, 1996 to Fujita, the disclosure of which is incorporated herein by reference, provides another substantial advance in filtering control technology. This patent discloses an EDFA that automatically adjusts the central wavelength of a tunable filter, positioned downbeam from a doped fiber amplification element, to trace the wavelength of the input signal component of an amplified signal.
The device provides many advantages, but it has a complex structure and relies upon specific features found in the output signals of typical current fiber amplifiers to achieve its objectives. In particular, it uses a two level wavelength sweep procedure: in a broad sweep, it locks onto a desired wavelength in the amplified signal by finding the wavelength where a negative peak occurs in the second derivative of the amplified signal intensity. In a narrow sweep, the intensity peak locked from the broad sweep is traced by repeatedly sweeping a narrow band to find the wavelength therein at which the first derivative of the intensity vanishes. This approach has undeniable elegance, but it requires both first and second order differentiators and also relies upon the amplified signal having a well-defined, single-peak feature corresponding to the input light signal. If the input signal is degraded, then the tests this system uses in its double sweep procedure may not provide reliable tracing results.
U.S. Pat. No. 5,572,351, entitled "Optical Communications Systems" and issued Nov. 5, 1996 1996 to Hadjifotiou, the disclosure of which is incorporated herein by reference, also provides an advanced wavelength tracing system for an EDFA having a tunable filter. The disclosed system automatically adjusts the central wavelength of the tunable filter by adding a pilot signal to the data signal at a wavelength spaced apart (above or below) the band occupied by the data signal. The frequency of the pilot component of the received signal is detected by the EDFA and the central frequency of the data band in the received signal is deduced from the received pilot frequency. This ingenious system also has certain limitations, such as requiring use of a pilot signal and relying upon comparable dispersion in the pilot signal and the data signal.
These systems, while providing significant advances, have certain limitations from which I have concluded that a wavelength-controlled EDFA with further improvements is needed. Such a system should provide robust wavelength tracing control without requiring special signal transmission formats. It should utilize only thoroughly proven control system components and should not rely upon specialized optical devices for operation. It also should not rely upon specific input signal features that may not be attainable in suboptimal operating situations.